Some reviews have understated the problems here. Marylin's cameo is not even one minute long - it's more like 30 seconds, and not very memorable. In fact, Marylin is completely upstaged by female lead Vera-Ellen (someone I'd never heard of), who has a very sexy dance number.
Not only does Groucho appear very little, but until the very end he does not interact with his brothers. Apparently they decided to add him at the last minute and shot several solo scenes featuring him that are interspersed throughout the film. This is unfortunate because Groucho's persona, in my opinion, was the center of the Marx brothers act. Harpo's antics grow wearisome in extended doses, and Chico is even more limited. Groucho, after all, was the one who always dropped cynical comments about the mediocre romantic storylines that were attached to the later Marx films, and thus made it possible to endure those storylines. Without his presence here, we are overwhelmed with the shabby, sentimental story about a theatrical troupe trying to put on a Broadway show.
There is one thing to recommend the film - it has Chico's most amusing piano number (after the one in Animal Crackers), as he attempts to improvise a duet with a violinist.